1. Field of the Invention
The present invention relates to an off-chip apparatus and a method for driving continuously and controlling the flow speed of fluid in a microfluidic chip. It is applicable to the field of microfluidic technology.
2. Description of Related Art
In recent years the development of microfluidic chips has earned a lot of attention due to the ability to integrate electronic, chemical and biomedical technologies to the chip. Microfluidic chips are also applicable to a wide range of fields such as pharmaceutical research, genetic engineering, gene expression, sequencing, protein assays, environment monitoring and clinical diagnosis. Advantages associated with microfluidic chips include the reduction of experimental error from inaccuracies in operation, the enhancement of system stability, the reduction of sample volume required, and the saving of time and labor.
The operation of a microfluidic chip often requires an active driving apparatus to move the fluid in the chip at a flow speed within a specified range. In the design of the driving apparatus, some features for microfluidic applications must be considered:    1. The amount of the fluid to be handled is very small, often at the nano- or micro-liter level. Therefore, the active driving apparatus must move the fluid with small positive or negative pressure.    2. The flow speed of the fluid driven must be controlled within a specified range. If the flow speed of the fluid is too fast or too slow, the microfluidic chip may not perform its function properly. In a bioassay chip, for example, if the fluid is driven too fast, the analyte in the fluid may leave the reaction zone before the necessary reactions are completed. Therefore, in addition to the need for low driving pressure, the apparatus must also provide design variables that may be customized to vary flow speed as needed for different applications.    3. The apparatus must provide a sufficient driving duration in whole process when moving the fluid. Again consider the bioassay chip example. The driving apparatus must continuously move the liquid sample at a flow speed within a specified range during the whole process to complete the necessary reaction on the chip. Often a bioassay may take seconds to minutes for completion.    4. Product cost. Microfluidic chips have a wide range of applications. Because, in bioassay, the parts are often disposable, they must be inexpensive.
Technologies used to drive the fluid on a chip are often divided into two categories. One is an off-chip independent pump, often larger than the chip and attached to it. The other is an on-chip micro driving mechanism. The off-chip independent pump can be one of several types: diaphragm, bellows, centrifugal, drum, flexible impeller, gear, hose, peristaltic pump or syringe pump. When the volume of the liquid to be driven is small, a syringe pump or peristaltic pump may be applicable. Although both pumps meet the requirements of driving fluid in a microfluidic chip, they may be expensive.
There are many types of on-chip micro pumps: bubble pumps, membrane pumps, diffuser pumps, rotary pumps, electrohydrodynamic pumps, electrophoretic pumps or ultrasonic pumps. Although on-chip micro pumps may meet the requirements for liquid volume, flow speed control and driving duration, one major disadvantage is that they often limit the choice of the material used for the microfluidic chip. Most on-chip micro pumps use silicon as a substrate, which requires photolithography as part of the manufacturing process. In many cases, additional parts, such as electrodes made from metal layers, magnetic coils made from special metals, or activating devices made from piezoelectric materials, are needed to make an on-chip micro pump. Such parts limit the choices of chip materials and increase the manufacturing cost of the product. In addition, the complexity of the manufacturing process of such parts leads to challenges in reproducibility of product quality.
In U.S. Pat. No. 6,802,228 an electro-mechanical device, a complicated mechanism, is used to control the syringe pump and drive the fluid. In U.S. Pat. Nos. 6,418,968, 6,748,978 a porous layer embedded in the chip as a valve to control the flow of the fluid limits the choice of material for the chip. In US Application No. 2002/0072719 the design requires collecting body fluid in a syringe. In U.S. Pat. No. 5,944,698 a syringe is designed to release liquid one drop at a time. Because the syringe must be filled with liquid before use, it may not be very convenient for certain microfluidic applications.
Therefore, the following features are desirable in an off-chip fluid driving apparatus: a mechanism based on a simple design, the ability to drive small quantities of liquid, flow speed within a specified range, sufficient driving duration, low manufacturing cost and simplicity in driving operation.